The advent of acid etching techniques in dentistry has opened new alternatives to existing methods in the field of dental prosthesis. The promise of securing the pontic in place with adhesive forces rather than by the means of mechanical retention appeared to offer some significant advantages over both fixed and removable dental prosthesis. The fixed prosthesis technique requires irreversible operational preparation of the abutment teeth involving destruction of tooth enamel. The fixed prosthesis techniques are also very costly and require a significant time involvement on the part of the patient, dentist and technician. The incidents of post-operational abutment teeth sensitivity, loss of their vitality and incidents of caries developing under cemented dentine are not infrequent consequences of fixed bridge preparations.
Removable prosthetic applicances are even more objectionable. For instance, they are generally functionally and esthetically inferior, uncomfortable and cause damage to the supporting teeth. Their use impairs oral hygiene and their safety is questionable.
An adhesive technique that relies on bonding of self-cured acrylic resin to phosphoric acid etched tooth enamel could provide a conservative, inexpensive, painless and comfortable solution for making partial dentures. Moreover, the same technique appeared to be promising for wireless stabilizing of teeth, for example, in periodontic or post-orthodontic treatment.
Attempts to make an adhesive pontic utilizing acid etching techniques were rewarded, however, with a limited success. One of the possible reasons for these failures was inadequacy of the adhesive. With the exception of the materials described in U.S. Pat. No. 4,172,323, materials specifically used for bonding pontics were not developed and, therefore, not suitable for this particular purpose.
In the reported clinical cases, a common composite restorative material or system comprised of composite restorative and liquid sealer was used in conjunction with phosphoric acid etching solutions. Note, for example the following publications:
(1) Leonard L. Portnoy, "Construction A Composite Pontic in a Single Visit", Dental Survey, August, 1973, pp. 20-23; PA1 (2) Sherwood S. Tucker, "Repair of a Loosened Pontic Replaced by Acid Etching", Dental Survey, July, 1974, p. 44; PA1 (3) Susan A. McEvoy and John R. Mink, "Acid-Etched Resin Splint for Temporarily Stabilizing Anterior Teeth", Journal of Dentistry for Children, November-December, 1974, pp. 439-41; PA1 (4) Paul M. Lambert, David L. Moore and Harry H. Elletson, "In Vitro Retentive Strength of Fixed Bridges Constructed with Acrylic Pontics and An Ultraviolet-light Polymerized Resin", J.A.D.A., Vol. 92, April, 1976, pp. 740-43; PA1 (5) Robert L. Ibsen, "Fixed Prosthetics with a Natural Crown Pontic Using an Adhesive Composite.", J.S.C.D.A., Vol. 41, 1974, pp. 100-102; PA1 (6) Richard I. Vogel, "The Use of a Self-Polymerizing Resin with Enamel Etchant for Temporary Stabilization", J. Periodontal., February, 1976, pp. 69-71; PA1 (7) A. Stuart, "An Unusual Space Maintainer Retained by an Acid-Etched Polymer Resin", British Dental Journal, Dec. 3, 1974, pp. 437-38; PA1 (8) Lee Pharmaceuticals Technical Bulletin #9090-1 (by Jan A. Orlowski and Robert Elwell), "Use of Restodent Dental Restorative in Fixed Bridge Prosthesis as a Long Term Temporary Space Maintainer", Apr. 20, 1973; PA1 (9) Abraham M. Speiser, "Transitional Splinting with Adhesive Material", Journal of the New Jersey Dental Association, February, 1974, pp. 34-35.
The entire disclosures of these publications are incorporated herein by reference and relied upon.
In these clinical cases, the etching procedure remained the same as used for restorative applications that are much less demanding when it comes to bonding strength. Moreover, the composite restorative materials lack flexibility and crack resistance, the characteristics most desirable for an adhesive used for bonding the pontic. During chewing, the teeth move in relation to each other. This imposes the requirement of flexibility, and fatigue resistance on the material used as an adhesive, making the ordinary restoratives unsuitable for this purpose.
U.S. Pat. No. 4,172,323, the entire disclosure of which is incorporated herein by reference and relied upon, describes a system for bonding pontics to the abutment teeth using a plastic screen in order to increase the force required to dislodge it. A methacrylate self-cured type adhesive of a non-composite type used in this system had the ability to bond to the reinforcing screen. The increase in the bonding strength achieved by the use of the screen was significant. However, it was not necessarily sufficient especially for posterior pontics. Moreover, it should be remembered that the reason for failures in adhesive type pontics is not necessarily insufficient bonding strength to etched enamel but most often inadequate mechanical properties of the adhesive material and its limited chemical resistance to the oral environment. It appears that in those respects the discussed patent offered little improvement over the prior art.
Bonding to human enamel relies on a number of tiny tags of cured adhesive extending into the crevices of etched enamel surface. Taking into consideration the uneven distribution of stresses during chewing and jaw movement, the properties of the adhesive such as flexural strength, impact resistance and elasticity become of prime importance, especially for posterior pontics where greater movements and greater mastication forces are involved. Highly cross-linked methacrylate type resins constituting the backbone of the adhesives used for bonding pontics were generally too stiff and too brittle for these applications. The chemical resistance to oral fluids was another important factor at least with some of these adhesive materials.